A common requirement of SAR is to detect relatively small objects in a generally featureless region, e.g. a ship on the ocean. In this situation, the radar footprint desirably has a wide swath, in the “cross track” direction at right angles to the “along track” direction of the radar platform, so that as large an area as possible is scanned. As will be explained, and as illustrated in FIG. 1, a wide swath requires a low pulse repetition frequency (PRF). Operating a SAR at a low PRF is likely to introduce ambiguities in the along track direction. These are “ghost” images that arise in adjacent lobes and are analogous to the grating lobes seen when monochromatic light is projected through a coarse diffraction grating.
When operating in SAR mode, the radar is usually required to pulse at a high rate (see FIG. 1), namely at a rate sufficiently high that when samples are taken along the trajectory followed by the radar (which is the position of the aperture that will be synthesised), the inevitable grating lobes that exist for sample spacing greater than 1 wavelength are adequately attenuated by the along-track pattern associated with the real aperture propagating pulses from and to the radar. This criterion is the primary driver to the PRF at which the radar must operate. The PRF is a function of the real aperture length (as that length gets shorter, so the real beam widens and has the potential to include more grating lobes so the PRF has to rise in order to introduce more samples and increase the angular spacing between those grating lobes) and the velocity with which the platform moves (low Earth orbiting satellites move at around 7500 m/s). A good working value for the spacing between samples is that this should be somewhat less than half the length of the real aperture.
A consequence of high PRF, is sensitivity to ambiguous returns from the across track direction. All measurements of distance from radar to target are made, modulo the spacing between pulses. So, for unambiguous performance in measuring distance from the radar, the distance between pulses should be greater than the distance on ground/surface that is illuminated by the across-track width of the radar beam. This implies use of a low PRF. However a low PRF then introduces ambiguities in the along track direction.
U.S. Pat. No. 5,745,069 proposes elimination of ambiguity considerations by using a transmitter format providing distinguishable sub-pulses, such as frequency division or code division format. However this is a complex solution requiring increased signal processing overhead and has the potential to introduce blind zones in the image associated with the high PRF pulse transmissions causing saturation in (or avoiding by blanking of) the receive chain.